This invention relates to hydraulic lash adjusters for overhead cam, internal combustion engines, and more particularly, it concerns an arrangement for the control of the internal operating pressure of such hydraulic lash adjusters.
It is common practice in the internal combustion engine art to employ an overhead cam arrangement to increase engine speed and operating efficiency. An overhead camshaft design increases the efficiency of valve train operation since the cam lobes bear directly upon the rocker arms which actuate the intake and exhaust valves. Engine speed may be increased due to the reduction in reciprocating weight resulting from the removal of push rods employed with conventional valve train arrangements. In order to insure quiet operation as well as relatively long life, the tolerances between the various moving parts of an overhead camshaft-type arrangement must be kept within fairly exact ranges. Any increase in the lash between the cam lobes and the follower surfaces on the rocker arms will result in noisy engine operation as well as increased wear of the moving parts.
Hydraulic lash adjusters have increasingly been used to compensate for cam surface and rocker arm wear. Under certain operating conditions, it has been found that the hydraulic lash adjuster plunger may extend or pump-up thereby opening the engine valves and interrupting engine operation. This pump-up action may occur if the engine is run at a relatively high speed prior to proper warm-up when the engine oil is cold and viscous or this condition may occur if the oil pump relief valve malfunctions. In these situations, excessive engine oil pressure may be present causing the hydraulic lash adjuster plunger to extend or pump-up. In a typical engine, the normal lubricant pressure in the gallery or lubrication passages may be within the range of 50-75 psi. Under the abnormal conditions described above, the oil pressure may exceed 100 psi.
In a conventional push rod actuated valve trains, the rocker arm ratio between the engine valve, the rocker arm pivot point and the push rod will multiply the valve spring load and thereby resist hydraulic tappet pump-up. With overhead cam valve trains, however, the rocker arm ratio is reversed and in inherent resistance to pump-up is not present.
U.S. Pat. No. 3,838,669 to Morris V. Dadd entitled "HYDRAULIC LASH ADJUSTER", issued Oct. 1, 1974 is an example of a lash adjuster construction having provision for reducing the pressure of the oil or lubricant in the lash adjuster oil reservoir. By reducing the internal or operating oil pressure of the lash adjuster, extension or pump-up of the plunger during abnormal conditions is prevented. In this arrangement, a cup-like element positioned in the plunger cavity serves to reduce the pressure in the oil reservoir by forming a restricted passageway.
Although functioning to reduce the internal operating pressure to acceptable levels, certain manufacturing and production problems are inherent with this arrangement. For example, it is difficult to obtain the precise tolerances required between the cup-like element and the inner portions of the plunger. Further, since a separate element is employed, certain assembly problems are present.
Therefore, it can be seen that a need exists for a hydraulic lash adjuster including means for reducing the internal operating pressure below that of the engine gallery pressure and whereby the problems heretofore experienced may be substantially alleviated.